Sediment Deposition
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Increase sediment deposition rate
Data collection in progress
Kakheti Steppes
Georgia
Problem: The natural migration of sedimentary material along the Iori river has been hindered by the Dali dam. Water with very little suspended sediment flows through the dam’s spillway, while the sediment transported by the river accumulates in the Dali reservoir, resulting in a lack of sediment transport to the river below the dam. This sediment is crucial for providing nutrients to the river beneath the dam, as well as replacing material that gets washed away by the flow of the river. Consequently, the lower part of the river has undergone drastic erosion. This in turn impacts species and communities living in aquatic habitats, as well as the extent and condition of floodplain forest that depends on periodic flooding from the river.
Action: SABUKO is working with the dam managers to recreate flooding events by periodically releasing water containing suspended sediment from the dam. They are also constructing gabions to retain water in the floodplain area below the dam when water is released, which will help to reduce flow rates and allow suspended sediment to be deposited on the riverbed, counteracting erosion.
Indicator: The amount of sediment in the water above and below the dam will be monitored so SABUKO can learn how much sediment the river naturally carries, how much is reaching the river downstream of the dam, and how their interventions are affecting downstream sediment deposition over time.
Sieve used to separate sediment particles of different sizes through a series of graded mesh layers. Photo: Giorgi Guliashvili.
Different sizes of sediment particles sorted by sieving. Photo: Giorgi Guliashvili.
Methods
SABUKO took five samples in one place above and below the Dali dam (see map below, Fig. 1) for their measurements. Sites were selected due to their ideal locations in representing natural river flow (e.g. where there is absence of any structures such as piers that would affect the local hydrological dynamics). The five samples were taken at different vertical heights at a single cross-section at one site upstream of the dam, and one site downstream of the dam.
Figure 1: Map of sampling sites above and below the dam. The scale shows distance in kilometres.
At both sites, they collected water flow and suspended sediment measurements. Water samples were taken in December 2023, March 2024 and June 2024, and sampling will continue throughout the project duration. Water discharge was sampled using a current meter. This was combined with measurements such as water level, mean water velocity, river cross-section width, and river depth to calculate flow rate. Suspended sediment and fluvial deposits were collected using a Niskin bottle, the contents of which were then filtered and dried in a laboratory to determine the amount of suspended load and its fractal dimension.
Results
Baseline results revealed much more suspended sediment upstream of the dam than downstream of the dam (Fig.2).
Figure 2: Suspended sediment upstream and downstream of the dam.
Table 1 below shows the composition of fluvial sediment at both sites. The majority (75%) of the sample taken upstream of the dam was comprised primarily of cobbles, or stones, of differing sizes, with a smaller percentage comprised of finer sediment such as gravel and sand. This was in contrast to the sample downstream of the dam, which was comprised primarily of smaller sedimentary material such as sand, some gravel, and silt. The overall total sediment load upstream was twice as high as the total load downstream.
Table 1: Fluvial composition across different sediment materials and size classes, measured upstream and downstream of the dam.
Baseline discharge rates were higher above the dam (8.83 m3/sec) compared to below the dam (6.55 m3/sec), meaning a greater volume of water was passing the sampling site upstream than downstream. Currently, dam releases occur approximately 2-3 times per year.
Each of these metrics will be collected 2-3 times each year until 2027, corresponding to periods of low water and flooding. SABUKO will use these data to assess if the measurements downstream of the dam improve over time in response to their restoration actions.
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